In producing and cleaning electronic parts, media specially prepared for these purposes conventionally include, for example, sulfuric acid, hydrofluoric acid, hydrogen peroxide and hydrochloric acid. These cleaning media continue to be suitably used depending on the intended application. However, these cleaning media are obtained by specially purifying corresponding products produced through chemical processes. The purification operations are complicated because they involve the step of removing metallic ingredients which have been introduced into the chemical products, for example, from the catalysts used in producing the same. As a result, the purified products are expensive. In addition, even if the purification operations are carefully conducted, the thus-purified products cannot always satisfy the reduction in allowable impurity levels required by advances in electronic devices. New substitute techniques are hence desired.
One of these substitute techniques is the use of ozonized water. In particular, highly ozonized water produced by electrolysis is known to be exceedingly effective, e.g., in cleaning electronic devices. However, because the use of ozonized water alone is insufficient in some cases, there is a growing need for a treatment liquid which has one or more functions not possessed by ozonized water, e.g., an oxidizing function and a reducing function, and which furthermore does not contain metallic elements.
Among such treatment liquids is so-called acidic water or ultra-acidic water. Acidic water generally has a pH of 3 or lower and an oxidation-reduction potential (ORP) of 1.2 V or higher and hence has an oxidizing ability. Consequently, the acidic water has the effect of, for example, decomposing organic substances or dissolving metallic deposits therein to remove these impurities, and has come to be used for the cleaning of electronic devices, etc.
Simultaneously with the production of the acidic water in an electrolytic cell, alkaline water having a pH of 10 or higher and an ORP of 0 V or lower is produced as a by-product in the cathode chamber of the electrolytic cell. Use of this alkaline water for cleaning, etc., has reached a stage of practical use.
These modified acidic and alkaline waters (electrolytic active waters or electrolytic ionic waters) have the same cleaning effect as reagents such as high-purity acids, alkalis, and hydrogen peroxide. Since the electrolytic active waters are markedly inexpensive, a considerable cost reduction is attainable.
In the electrolytic production of acidic water and alkaline water, a two-chamber type electrolytic cell is generally used which is partitioned into an anode chamber and a cathode chamber with an ion-exchange membrane serving as a diaphragm. For conducting electrolysis using this type of electrolytic cell, an appropriate supporting electrolyte is added to electrolyte liquids in order to impart ionic conductivity thereto. However, the cleaning waters thus produced mostly contain the supporting electrolyte remaining therein or are contaminated with metallic ions and particles which are attributable to the dissolution of material constituting the inner wall of the electrolytic cell main body by the electrolytic solutions. If such contaminated cleaning waters are used for cleaning electronic devices such as semiconductors and liquid crystals, metallic ions and other contaminants contained in the cleaning waters adhere to semiconductor surfaces and these adherent impurities cause problems such as insulation failures.
Consequently, for producing high-purity acidic water and alkaline water for use in, e.g., the cleaning of electronic devices, electrolysis is conducted using electrolytic liquids not containing an electrolyte dissolved therein and using an ion-exchange membrane so that the membrane itself functions as a solid electrolyte. In this method, when ultrapure water is used as an anolyte and a catholyte, almost no impurities are introduced into the anolyte and catholyte used as feedstocks or into the acidic water and alkaline water thus produced. Namely, the desired high-purity acidic water and alkaline water can be produced.
In the above electrolysis, the efficiency of producing acidic water (anode water) in the anode chamber and the efficiency of producing alkaline water (cathode water) in the cathode chamber vary depending on the kind of feedstock water, the kind of electrode catalyst, and electrolytic conditions including the current density. Furthermore, the required amount of acidic water and alkaline water vary depending on the intended purpose. Consequently, when a single electrolytic cell (which may be either a two-chamber or three-chamber cell) is used for simultaneously yielding acidic water and alkaline water, one of the waters tends to be produced in excess and this causes a cost increase. Although it is desirable to suitably regulate the production amount of acidic water and alkaline water as needed, this regulation is virtually impossible in a conventional apparatus.
In a conventional electrolytic cell for use in producing high-purity acidic water and high-purity alkaline water, electrolysis is conducted while adding water, especially ultrapure water, to the single electrolytic cell on the anode side and the cathode side as needed to thereby simultaneously obtain acidic water and alkaline water in the single electrolytic cell. This technique generally yields alkaline water in excess, and the excess alkaline water is discarded. However, when ultrapure water, in particular, is used as an electrolyte liquid, it is exceedingly uneconomical to discard the excess product, e.g., alkaline water, because ultrapure water itself is very expensive and much time is required to produce the same.
Furthermore, the conventional electrolytic cell described above has an unsolved problem in that the electrode material moves, although in a slight amount, through the ion-exchange membrane to the counter-electrode side to reduce the purity of the electrolytic water thus produced. Because this dissolution of electrode material occurs in an amount of about several tens of ppb at the most, contamination therewith poses no problem in ordinary applications for which feedstock water having a purity on the same level as ion-exchanged water is used. However, contamination with the electrode material exceeds the allowable impurity levels for acidic or alkaline water for electronic-device cleaning.